The present invention is directed to semiconductor devices with nested rows of contacts and to a method of making such semiconductor devices.
Semiconductor devices, such as integrated circuits, comprise a semiconductor die (or chip) in a package with exposed electrical contact surfaces. The completed devices may be mounted on a support with electrical connections, such as a printed circuit board (PCB), for example. Using surface mount technology the electrical contact surfaces of the package can be soldered directly to corresponding pads on the support, providing mechanical attachment as well as electrical connections.
A completed surface mount device typically includes an electrically insulating molding material that covers the semiconductor die such that the device presents a top face and a bottom, active face, which are generally rectangular or square, and transversely extending edges. The molding compound may encapsulate the semiconductor die completely, or may define an air cavity that is then sealed with a ceramic or plastic lid. Typically, the device has a pair of sets of electrical contact surfaces on opposite sides of the device (‘dual in-line package’) or two orthogonal pairs of sets of electrical leads on respective sides of the device (‘quad package’).
In one type of package, the electrical contact surfaces are positioned in the bottom active face of the device. Each set of electrical contact surfaces includes discrete elements disposed side by side at intervals in rows in the active face of the device for soldering to the electrical connections of the support. In order to increase the number of contact surfaces available, more than one row of electrical contact surfaces may be provided in each set on the respective side of the device. The adjacent rows at each respective side of the device are nested, extending parallel to each other and to the adjacent side of the device, one row being further from the adjacent side of the device than the other row is.
The semiconductor die may be mounted in the device on a pad or flag of the same material as the electrical contact surfaces, which is usually a metal, such as copper, which may be plated. The die pad may be exposed at the bottom face of the device, to assist cooling the die, known as an exposed-pad package. Alternatively, the die pad may be omitted, known as a non-exposed pad package. In a non-exposed pad package the die may be mounted on the discrete electrical contact elements. In each case, the die and electrical contact elements and any die pad are held together mechanically by the encapsulating molding material. The electrical contact elements of the device may be connected electrically to electrical contact pads on the die itself by bonded wires, of gold, copper or aluminum for example, accommodating differential thermal expansion of the die and the package materials.
A prevalent technique used in manufacturing such a surface mount device includes forming an array of lead frames in a strip or sheet of electrically conductive material, usually metal, by etching and/or stamping for example. Each lead frame comprises a frame structure common to adjacent lead frames and supporting in the array the sets of discrete electrical contact portions which will form the sets of electrical contacts of the completed device after singulation and any die pad for mounting the die. The array of lead frames could comprise a single strip but typically comprises a two-dimensional array, with the supporting frame structure of the complete array comprising surrounding bars on the outer edges of the array and intersecting intermediate bars common to adjacent lead frames.
In a typical surface mount semiconductor device packaging process using lead frames, the semiconductor dies are mounted on and connected electrically to respective ones of the lead frames. The encapsulation material is then molded over and around the lead frame strip or sheet, possibly with a lid in the case of an air cavity package, so as to encapsulate the integrated circuit dies, the electrical contact surface elements and the bonded connection wires of each of the lead frames. The individual devices are then separated by a singulation process, in which the lead frame strip or sheet is cut apart. The singulation may be a saw operation. If desired, saw singulation enables the molding compound to be applied over the entire array, being cut subsequently during the singulation process. During saw singulation, a saw blade is advanced along ‘saw streets’ which extend between the electrical contact surface elements of adjacent lead frames, so as to cut off the supporting frame structures of the lead frames from the electrical contact surface portions of the lead frames and separate the individual devices from each other.
A high level of quality control of production process, including the singulation process, is desirable.